The field of the invention is programmable controllers such as those disclosed in U.S. Pat. Nos. 3,942,158 and 3,810,118 and copending patent application Ser. No. 790,251.
Programmable controllers are typically connected to industrial equipment such as assembly lines and machine tools to sequentially operate the equipment in accordance with a stored control program. In programmable controllers such as those disclosed in the above cited patents and patent application, for example, the control program is stored in a memory and includes instructions which are read out in rapid sequence to examine the condition of selected sensing devices on the controlled equipment and instructions which energize or deenergize selected operating devices on the controlled equipment contingent upon the status of one or more of the examined sensing devices.
The processor in a programmable controller is designed to rapidly execute programmable controller type instructions which call for the manipulation of single-bit input data and the control of single-bit output data. The length of the control program, and hence the complexity of the system to be controlled, is limited to insure that the entire control program can be executed, or "scanned," within a set time. Such time limits are required to insure that the programmable controller will rapidly scan all of the sensing devices and set all of the operating devices on the controlled system and to thereby promptly respond to any change in the status of the controlled system. Of course, the speed at which a controller processor can execute programmable controller instructions has a direct bearing on the size of the machine or process which it can effectively control and, therefore, it is a primary objective in this art to increase the instruction execution rate of the controller processor. Nevertheless, for any given controller processor, there is a finite limit on the size of the control program and the response time of the controller.
Some industrial applications of programmable controllers include a few I/O devices which require considerably faster service from the programmable controller than the remainder of the system devices. For example, a programmable controller which scans all I/O devices at least once each twenty milliseconds is considered fast enough for the vast majority of applications and hence, this is considered a reasonable design criteria for a programmable controller. However, in some installations containing hundreds, or even thousands, of I/O devices the controlled system may contain a handful of sensing devices and operating devices which require rapid service in ten milliseconds or less. A programmable controller with a twenty-millisecond scan time is thus not suitable and a special purpose hardwired controller, or computer control system, is usually employed in such installations.